Clutch for pivoting workpiece support

ABSTRACT

A clutch and a workpiece support incorporating the same selectively restrict rotation of a rotatable member through the use of one or more rollers that are pinched between the outer surface of the rotatable member and corresponding convergent surfaces disposed on a housing opposing such outer surface. In effect, each roller is interposed between a surface on a fixed housing and an outer surface of a rotatable member such that rotation of the rotatable member rolls the roller toward a converging gap to restrict further rotation of the rotatable member. A biasing mechanism biases the roller toward the converging gap, and a release mechanism is utilized to quickly and efficiently decouple the roller from the rotatable member by biasing the roller in a direction away from the converging gap and thereby preventing the roller from inhibiting rotation of the rotatable member.

FIELD OF THE INVENTION

The invention is related to pivoting workpiece supports and adjustmentmechanisms for use therewith.

BACKGROUND OF THE INVENTION

Pivoting workpiece supports are utilized in a number of manufacturingenvironments to provide a variable orientable work surface for theassembly of components such as printed circuit assemblies, computer harddrives, automotive components, etc.

With a pivoting workpiece support, a platform is pivotably supported ona base, with a rotational coupling utilized to secure the platformsupport to the base. An adjustment mechanism may also be coupled to theworkpiece support to permit an operator to adjust the relative angle ofthe platform.

One benefit of a pivoting workpiece support is the fact that theplatform can be adjusted for use with different operators, or may beadjusted by a single operator to optimize the orientation of theplatform for different tasks. In addition, a pivoting platform may alsobe used to permit multiple sides of a workpiece to be accessed at acomfortable orientation for an operator.

To maximize an operator's productivity when using a workpiece support,it is desirable for the adjustment mechanism to be as fast and easy tooperate as possible. Also, given that an operator can exert significanttorque while working on a platform, it is also desirable for anadjustment mechanism to be secure and reliable to minimize unwantedmovements of the platform.

Conventional adjustment mechanisms such as set screws have been used toselectively lock a platform in a desired rotational orientation forextended periods of time. With a set screw, a threaded screw is orientedperpendicular to a pivot shaft for radial movement relative to the axisof rotation of the pivot shaft. Rotation of the threaded screw applies aforce through the end of the screw to the pivot shaft to restrictrotation of the pivot shaft about its axis of rotation.

One problem with set screws and the like is that they are oftendifficult to manipulate in a fast and efficient manner. Some set screwsmay require several turns both to release and re-secure the pivot shaft.In addition, the force required to twist a set screw may induce fatiguein an operator, particularly if the operator makes frequent adjustments.As a consequence, set screws are not typically used in environmentswhere repetitive adjustments are required.

One conventional adjustment mechanism utilized in some pivotingworkpiece supports is a pawl and ratchet design, where a toothed wheelcoupled to a pivot shaft is engaged by a pawl to selectively permit orrestrict rotation of the pivot shaft. Other mechanisms utilize aspring-loaded shot pin configured to engage in one of a plurality ofholes disposed circumferentially around a shaft. Conventional pawl andratchet designs and shot pin designs, however, are not infinitelyadjustable, and are restricted by the number of teeth disposed on theratchet or holes disposed on a shaft. Such designs may also be slow andcumbersome, and may require significant effort to operate.

Therefore, a significant need continues to exist in the art for a fastand efficient adjustment mechanism for use with pivoting workpiecesupports and the like.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art by providing a clutch and a workpiece support incorporatingthe same in which rotation of a rotatable member is selectivelyrestricted through the use of one or more rollers that are pinchedbetween the outer surface of the rotatable member and correspondingconvergent surfaces disposed on a housing opposing such outer surface.In effect, each roller is interposed between a surface on a fixedhousing and an outer surface of a rotatable member such that rotation ofthe rotatable member rolls the roller toward a converging gap torestrict further rotation of the rotatable member. A biasing mechanismbiases the roller toward the converging gap, and a release mechanism isutilized to quickly and efficiently decouple the roller from therotatable member by biasing the roller in a direction away from theconverging gap and thereby preventing the roller from inhibitingrotation of the rotatable member.

Consistent with one aspect of the invention, therefore, a clutch isprovided for use in selectively restricting rotation of a rotatablemember that rotates within a rotational plane and about an axis ofrotation. The rotatable member includes an annular engagement surfacecircumscribing the axis of rotation. Moreover, the clutch comprises ahousing having defined thereon a convergent surface opposing the annularsurface of the rotatable member and separated therefrom by a gap thatdecreases between first and second positions within the rotational planeof the rotatable member. The clutch also includes a roller disposedwithin the gap between the convergent and annular surfaces, the rollerconfigured to rotate about an axis of rotation that is parallel to theaxis of rotation of the rotatable member, and the roller having adiameter that is less than the gap between the convergent and annularsurfaces proximate the first position, but is greater than or equal tothe gap between the convergent and annular surfaces proximate the secondposition. The clutch further includes a biasing mechanism coupled to theroller and configured to bias the roller toward the second position towedge the roller between the convergent and annular surfaces and therebyrestrict rotation of the rotatable member, and a release mechanismcoupled to the roller and configured to selectively oppose the biasingmechanism to bias the roller toward the first position and permitrotation of the rotatable member.

Consistent with another aspect of the invention, a workpiece support isprovided comprising a base and a platform, with the platform pivotablycoupled to the base through an adjustment mechanism incorporating aclutch having the features described above to selectively permitpivoting of the platform relative to the base.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and of the advantages and objectives attained through itsuse, reference should be made to the Drawings, and to the accompanyingdescriptive matter, in which there is described exemplary embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary perspective view of a workpiece supportincorporating a clutch consistent with the invention.

FIG. 2 is a partially-exploded perspective view of the clutch of FIG. 1.

FIG. 3A is a cross-sectional view of the clutch of FIG. 2, taken throughlines 3A—3A thereof.

FIG. 3B is a cross-sectional view of the clutch of FIG. 3A, subsequentto disengaging the clutch.

FIG. 4 is an enlarged cross-sectional view of one of the rollers in theclutch of FIG. 2, illustrating the relative dimensions of the roller tothe converging gap within which the roller resides.

DETAILED DESCRIPTION

Turning to the drawings, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates a workpiece support 10having a platform 12 pivotably coupled to a base 14 through a pair ofupright supports 16 (only one of which is shown in FIG. 1). Platform 12includes opposing shafts 18, each engaging a corresponding bearing 20 inupright support 16 to permit pivoting of the platform about a pivot axis22.

Pivoting of platform 12 relative to base 14 is selectively enabledthrough the use of an adjustment mechanism 24 including a clutch 26engaging a rotatable member 28 including an annular engagement surface30, which in the illustrated embodiment has a profile of a rightcylinder. Further in the illustrated embodiment, the clutch 26 isfixedly mounted to upright support 16, while rotatable member 28 isfixedly mounted to platform 12. It will be appreciated that the clutch26 and rotatable member 28 may be reversed, and that rotatable member 28may include alternate engagement surface profiles than that illustratedin FIG. 1.

Platform 12 in the illustrated embodiment forms an integrated worksurface and assembly carriage, including first and second opposingframes 32, 34 joined at each end by a pair of hinges 36, 38. Frames 32,34 are separated from one another by a plurality of standoffs 40 securedat each corner of frame 34. The frames are selectively locked in theconfiguration shown in FIG. 1 by pivoting a latch 42 mounted on frame 34about a shaft 42 a to engage a catch 44 disposed on an upright support46 secured to frame 32. Latch 42 includes a slot 42 b that engages catch44, which is implemented as a roller in the illustrated embodiment.

When the latch 42 (as well as a corresponding latch at the opposite endof platform 12 (not shown)) is in the position illustrated in FIG. 1,frames 32 and 34 are capable of being pivoted away from one another,with the configuration of hinges 36, 38 (specifically the relativedistances between hinge points 36 a and 36 b, and 38 a and 38 b)selected to initially separate the frames in a direction generallyperpendicular to the planes of frames 32, 34, and then swing open inmuch the manner as a clam shell.

Each of frames 32, 34 is configured to receive a removable panel (notshown) secured by one or more clamps to, in effect, provide an assemblyclamp with which components can be assembled together on a printedcircuit board.

Adjustment mechanism 24 is illustrated in greater detail in FIGS. 2,3A-3B and 4. As best shown in FIG. 2, adjustment mechanism 24 includes aclutch 26 incorporating a housing 50 having a wall formed thereon at 52that cooperates with rotatable member 28 to define a chamber 53 withinwhich is retained a pair of rollers 54, 56. A plurality of bores 58 arealso provided in housing 50 to secure housing 50 to upright support 16of base 14.

Housing 50, rollers 54, 56 and rotatable member 28 may be constructed ofdifferent materials, e.g., hardened tool steel with a hardness of aboutR/C 58-60.

As best shown in FIG. 3A, housing 50 also includes a pair of bores 60,62 extending along the plane of rotation of rotatable member 28 torespectively retain a biasing mechanism 64, 66 that biases acorresponding roller 54, 56 toward a convergent gap within which theroller resides. Each biasing mechanism 64, 66 includes a pin 68 biasedby means of a spring 70, and retained within the respective bore 60, 62by a set screw 72. It will be appreciated that other biasing mechanismsmay be utilized consistent with the invention, including other springdesigns, as well as non-mechanical biasing structures incorporating theuse of pressurized fluid, electromagnetic force, etc. Moreover, eachbiasing mechanism may apply a bias directly to each roller, or maysupply a biasing force to other structures, e.g., an axle about whicheach roller rotates. Other modifications will be apparent to one ofordinary skill in the art having the benefit of the instant disclosure.

Clutch 26 also includes a release mechanism 74, implemented in theillustrated embodiment by a spring-loaded plunger including a wedge body76 mounted at one end of a shaft 78, with a handle 80 secured to theopposite end of the shaft and biased to a disengaged position by aspring 82. Housing 50 includes concentric bores 84, 86 that respectivelyreceive shaft 78 and wedge body 76.

In the illustrated embodiment, wedge body 76 has opposing engagementsurfaces 90, 92 that extend generally parallel to one another and in thedirection of actuation of release mechanism 74. The opposing engagementsurfaces are joined by an end surface 94 extending perpendicularthereto. Further, the edges 96, 98 along which end surface 94 engagesengagement surfaces 90, 92 are beveled, e.g., with a 45 degree chamfer.It will be appreciated that other wedge profiles may be utilized in thealternative, e.g., converging surfaces 90, 92; various edge bevels;e.g., rounded, mitered; etc.

In the illustrated implementation, release mechanism 74 is amomentary-type release mechanism, whereby no detent or other structureis provided to lock or otherwise hold release mechanism 74 in an engagedposition. In other embodiments, however, it may be desirable to providesuch additional functionality for the release mechanism to maintain theclutch in a disengaged configuration. Also, release mechanism 74 may bebiased toward an engaged position in other embodiments so the rotatablemember 28 is normally free to rotate.

As discussed above, FIG. 3A illustrates clutch 26 in a disengagedconfiguration, with release mechanism 74 disengaged, and as aconsequence, rotatable member 28 prevented from rotating relative to theclutch. Each roller 54, 56 is retained within a converging gap 100, 102.

As best shown in FIG. 4, gap 100 is defined between a convergent surface104 defined on wall 52 of housing 50 and annular surface 30 of rotatablemember 28. The gap 100 converges in the direction of bias applied bybiasing member 64. The diameter D of roller is selected such that, whenrelease mechanism 74 is disengaged, the roller 54 is biased to aposition in the convergent gap where the diameter of roller 54 isgreater than or equal to the length of the gap (denoted at G₂). As such,roller 54 is placed in engagement with both surfaces 30, 104.

Now turning to FIG. 3B, upon actuation of release mechanism 74, wedge 76is interposed between rollers 54, 56, displacing these rollers to thepositions illustrated at 54′, 64′. Returning to FIG. 4, it may be seenthat when roller 54 is displaced to the position shown at 54′, thediameter D of the roller is less than the length of gap 100 illustratedat G₁. As such roller 54 does not simultaneously engage both of surfaces30, 104, and rotatable member 28 is thus free to rotate relative to theclutch.

It will be appreciated that the width of wedge body 76 is selectedrelative to the separation of the rollers to ensure that the wedge willdisplace each of the rollers when the release mechanism is engaged.

One important feature of the illustrated embodiment is that, when therelease mechanism 74 is not actuated and roller 54 engages both ofsurfaces 30, 104, torque applied to rotatable member 28 that urgessurface 30 in a direction toward the converging gap (clockwise in theillustrated embodiment), will urge roller 54 to rotate in a contrarydirection, which urges the roller into converging gap 100. The forcethat restricts further rotation of the rotatable member 28 thereforeincreases in proportion with the force applied to rotate the member.

It may be seen that rotation of rotatable member 28 in an oppositedirection will have an opposite effect, that of urging roller 54 to movetoward the position illustrated at 54′, thereby overcoming the biasapplied by biasing mechanism 64. However, with the correspondingstructure utilized for roller 56, such opposite movement will urgeroller 56 into its converging gap 102, thereby restricting rotation ofthe rotatable member 28 in the opposite direction. As such, thecombination of rollers 54 and 56 restrict all rotation of rotatablemember 28 relative to clutch 26.

It should be appreciated that, in other embodiments, it may be desirableto incorporate only one roller, thereby omitting the second roller. Suchimplementations may be desirable to provide one-way clutchingfunctionality.

In the illustrated implementation, the diameter of each roller 54, 56 isabout 0.375 inches, with the length of the gap at the engaged position(G₂) equal to about 0.375 inches. The length of the gap at thedisengaged position (G₁ where the roller engages the engaging surface ofwedge body 76), is about 0.390 inches.

The position at which the diameter of the roller equals the length ofthe gap is selected to ensure that the rollers are easily displaced bythe release mechanism when the clutch is in an engaged configuration.This is ensured in the illustrated embodiment by configuring convergentsurface 104 (and surface 106) to provide a gap that roughly equals thediameter of the roller when the roller is in a position where itstangent to the rotatable member forms an angle α with a tangent takenthrough the centerline of release mechanism 74, with α between about11.0 and 12.0 degrees, e.g., about 11.5 degrees in the illustratedembodiment. It is also to be noted that wedge 76 does project somewhatinto gap 102 when in a disengaged position to limit the amount that eachroller 54, 56 can be urged into the respective convergent gaps inresponse to torque applied to rotatable member 28.

Therefore, in the illustrated embodiment, clutch 26 is actuated byapplying a radial force to release mechanism 74 to displace rollers 54,56 outwardly relative to one another, thereby overcoming the biasmechanisms 64, 66 and freeing rotatable member 28 to rotate relative tothe clutch. Upon disengagement of release mechanism 74, spring 82 movesthe wedge body 76 to a disengaged position, thereby permitting rollers54, 56 to be biased inwardly relative to one another and into therespective converging gaps to restrict rotation of the rotatable member.

Returning to FIG. 1, in use, workpiece support 10 may be utilized in aprinted circuit board assembly process to assemble components on aprinted circuit board. A typical assembly process utilizing workpiecesupport 10 incorporates releasing latches 42 and separating frames 32,34 into an open configuration. Suitable panels are installed within eachof frames 32 and 34, including, for example, a padded surface on theunderside of the panel attached to frame 34, and an apertured panelretained by frame 32 to permit access to the underside of the assembly.A circuit board is placed onto the panel retained by frame 32 overlayingone or more apertures in the panel, and centered by means of suitablestructure disposed on the panel. Components to be attached to theprinted circuit board are placed on top of the circuit board inappropriate locations, and the frame 34 is then brought down to engagethe pad on the underside of the panel retained by the frame with thecomponents placed on top of the circuit board. Latches 42 are thenactuated to secure frames 32, 34 to one another with the components inthe printed circuit board assembly sandwiched together. It will beappreciated that standoffs 40 provide the appropriate separation for theparticular components being assembled.

Once frames 32, 34 are engaged to one another via latches 42, adjustmentmechanism 24 is activated by engaging release mechanism 74, and platform12 is then rotated about 180° to orient the underside of the printedcircuit board in a convenient location for an operator. Apertures in thepanel retained by frame 32 are then utilized to apply any fasteners orother components to the underside of the printed circuit board tocomplete the assembly process. Release mechanism 74 is then engaged asecond time, and platform 12 is rotated approximately another 180° toreturn the platform to its original orientation, and thereby permit theoperator to disengage latches 42, separate frames 32, 34, and remove theassembled components.

It will be appreciated that, by utilizing different panels in each offrames 32, 34, and by optionally replacing standoffs 40, a wide varietyof assemblies may be constructed using the same overall workpiecesupport hardware. As such, workpiece support 10 can be an extremelyflexible and versatile manufacturing tool suitable for use in a widevariety of applications.

It will also be appreciated that, during any of the aforementionedassembly steps, adjustment mechanism 24 permits infinite adjustabilityof platform 12 relative to base 14. As such, an operator can adjust theplatform as he or she desires to optimize the comfort and convenience ofthe device.

Various modifications may be made to the illustrated embodiments withoutdeparting from the spirit and scope of the invention. For example, whileconvergent surface 104 on housing 50 is illustrated as a planer surfacethat is parallel to a tangent of the rotatable member, it will beappreciated that various surface profiles that provide a converging gaprelative to rotatable member 28 may be utilized in the alternative.Moreover, while rollers 54, 56 are circumferentially oriented aroundrotatable member 28 to permit a single wedge body 76 to separate therollers, it will be appreciated that different circumferentialorientations of the two rollers may be utilized in other embodiments.Moreover, separate release mechanisms may be utilized for each roller ifdesired.

Each roller 54, 56 further need not occupy the same rotational plane ofrotatable member 28. Moreover, in the illustrated embodiment, eachroller is simply located within its respective converging gap, withoutany separate mechanism retaining the roller. In other embodiments,however, it may be desirable to support each roller on an axle andretain the ends of the axle to maintain the axis of rotation of theroller in parallel with that of the rotatable member. Moreover, whilethe convergent surfaces for the respective rollers 54, 56 areillustrated in the same plane, such surfaces may be different profiles.

Other modifications will be apparent to one of ordinary skill in theart. Therefore, the invention lies in the claims hereinafter appended.

What is claimed is:
 1. A clutch for use in selectively restrictingrotation of a rotatable member that rotates within a rotational planeand about an axis of rotation, the rotatable member including an annularengagement surface circumscribing the axis of rotation, the clutchcomprising: (a) a housing having defined thereon a convergent surfaceopposing the annular surface of the rotatable member and separatedtherefrom by a gap that decreases between first and second positionswithin the rotational plane of the rotatable member; (b) a rollerdisposed within the gap between the convergent and annular surfaces, theroller configured to rotate about an axis of rotation that is parallelto the axis of rotation of the rotatable member, and the roller having adiameter that is less than the gap between the convergent and annularsurfaces proximate the first position, but is greater than or equal tothe gap between the convergent and annular surfaces proximate the secondposition; (c) a biasing mechanism coupled to the roller and configuredto bias the roller toward the second position to wedge the rollerbetween the convergent and annular surfaces and thereby restrictrotation of the rotatable member; and (d) a release mechanism coupled tothe roller and configured to selectively oppose the biasing mechanism tobias the roller toward the first position and permit rotation of therotatable member, the release mechanism including a wedge movableradially with respect to the rotatable member between a disengagedposition and an engaged position proximate the roller.
 2. The clutch ofclaim 1, wherein the convergent surface is a first convergent surface,the gap is a first gap, the roller is a first roller, and the biasingmechanism is a first biasing mechanism, the clutch further comprising:(a) a second convergent surface disposed on the housing and opposing theannular surface of the rotatable member, the second convergent surfaceseparated from the annular surface by a second gap that decreasesbetween third and fourth positions within the rotational plane of therotatable member; (b) a second roller disposed within the second gapbetween the second convergent and annular surfaces, the second rollerconfigured to rotate about an axis of rotation that is parallel to theaxis of rotation of the rotatable member, and the second roller having adiameter that is less than the second gap between the second convergentand annular surfaces proximate the third position, but is greater thanor equal to the second gap between the second convergent and annularsurfaces proximate the fourth position; and (c) a second biasingmechanism coupled to the second roller and configured to bias the secondroller toward the fourth position to wedge the second roller between thesecond convergent and annular surfaces and thereby restrict rotation ofthe rotatable member.
 3. The clutch of claim 2, wherein the releasemechanism is further configured to selectively oppose the second biasingmechanism to bias the second roller toward the third position and permitrotation of the rotatable member.
 4. The clutch of claim 3, wherein thefirst and second convergent surfaces are circumferentially orientedabout the axis of rotation of the rotatable member, with the second andfourth positions interposed between the first and third positions;whereby the first and second rollers are configured to respectivelyrestrict rotation of the rotatable member in first and seconddirections.
 5. The clutch of claim 4, wherein the engaged position ofthe wedge is intermediate the first and second rollers.
 6. The clutch ofclaim 5, wherein the first and second convergent surfaces each extendgenerally parallel to a tangent of the annular surface.
 7. The clutch ofclaim 6, wherein the wedge includes first and second engagement surfacesseparated by a width that is greater than a separation between the firstand second rollers when the first and second rollers are biased to thesecond and fourth positions, respectively.
 8. The clutch of claim 7,wherein the first and second engagement surfaces are generally parallelto one another, wherein the wedge further includes an end surfaceextending perpendicular to the first and second engagement surfaces, andwherein the wedge is beveled along intersections of the end surface withthe first and second engagement surfaces.
 9. The clutch of claim 5,wherein the release mechanism further comprises a spring-loaded plungercoupled to the wedge and configured to bias the wedge toward thedisengaged position.
 10. The clutch of claim 9, wherein thespring-loaded plunger includes a handle.
 11. The clutch of claim 9,wherein the release mechanism is configured as a momentary releasemechanism.
 12. The clutch of claim 2, wherein the first and secondbiasing mechanisms respectively comprise first and second spring-loadedpins abutting the first and second rollers, respectively.
 13. The clutchof claim 1, wherein the roller is configured to roll along each of theconvergent and annular surfaces toward the second position in responseto torque applied to the rotatable member to rotate the rotatable memberin a direction from the first position to the second position.
 14. Aclutch, comprising: (a) a disk configured to rotate within a rotationalplane and about an axis of rotation, the disk including a rightcylindrical outer surface circumscribing the axis of rotation; (b) ahousing having defined thereon first and second convergent surfacesdefined in a common plane extending generally parallel to a tangent tothe outer surface of the disk, the first convergent surface separatedfrom the outer surface of the disk by a first gap that decreases betweenfirst and second positions within the rotational plane of the disk, andthe second convergent surface separated from the outer surface of thedisk by a second gap that decreases between third and fourth positionswithin the rotational plane of the disk, wherein the second and forthpositions are interposed between the first and third positions; (c) afirst roller disposed within the first gap between the first convergentsurface and the outer surface of the disk, the first roller configuredto rotate about an axis of rotation that is parallel to the axis ofrotation of the disk, and the first roller having a diameter that isless than the first gap proximate the first position, but is greaterthan or equal to the first gap proximate the second position; (d) asecond roller disposed within the second gap between the secondconvergent surface and the outer surface of the disk, the second rollerconfigured to rotate about an axis of rotation that is parallel to theaxis of rotation of the disk, and the second roller having a diameterthat is less than the second gap proximate the third position, but isgreater than or equal to the second gap proximate the fourth position;(e) a first spring-loaded pin extending in a direction generallyparallel to the first convergent surface and engaging the first rollerto bias the first roller toward the second position and thereby wedgethe first roller between the first convergent surface and the outersurface of the disk to restrict rotation of the disk in a direction fromthe first position toward the second position; (f) a secondspring-loaded pin extending in a direction generally parallel to thesecond convergent surface and engaging the second roller to bias thesecond roller toward the fourth position and thereby wedge the secondroller between the second convergent surface and the outer surface ofthe disk to restrict rotation of the disk in a direction from the thirdposition toward the fourth position; and (g) a momentary releasemechanism comprising: (i) a wedge including first and second engagementsurfaces extending generally parallel to one another and joined alongfirst and second beveled edges, respectively, by an end surfaceextending generally perpendicular thereto, the first and secondengagement surfaces separated by a width that is greater than aseparation between the first and second rollers when the first andsecond rollers are biased to the second and fourth positions; and (ii) aspring-loaded plunger extending radially toward the axis of rotation ofthe disk and having the wedge mounted thereto for radial movement of thewedge between a disengaged position and an engaged position intermediatethe first and second rollers, wherein the spring-loaded plunger isbiased to move the wedge toward the disengaged position, and wherein,when the wedge is positioned in the engaged position, the wedge opposesthe first and second spring-loaded pins to respectively bias the firstand second rollers toward the first and third positions to permitrotation of the disk about the axis of rotation.
 15. A clutch for use inselectively restricting rotation of a rotatable member that rotateswithin a rotational plane and about an axis of rotation, the rotatablemember including an annular engagement surface circumscribing the axisof rotation, the clutch comprising: (a) a housing having defined thereona convergent surface opposing the annular surface of the rotatablemember and separated therefrom by a gap that decreases between first andsecond positions within the rotational plane of the rotatable member;(b) a roller disposed within the gap between the convergent and annularsurfaces, the roller configured to rotate about an axis of rotation thatis parallel to the axis of rotation of the rotatable member, and theroller having a diameter that is less than the gap between theconvergent and annular surfaces proximate the first position, but isgreater than or equal to the gap between the convergent and annularsurfaces proximate the second position; (c) a spring-loaded pin abuttingthe roller and configured to bias the roller toward the second positionto wedge the roller between the convergent and annular surfaces andthereby restrict rotation of the rotatable member; and (d) a releasemechanism coupled to the roller and configured to selectively oppose thespring-loaded pin to bias the roller toward the first position andpermit rotation of the rotatable member.